Hardware buttons support

Add [Go Back](https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/back) command
Add [Execute Script](https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/execute) support for "mobile:" prefixed commands. See [list of supported commands](https://github.com/2gis/winphonedriver/wiki/Script-execution-support).